Recent efforts to improve the long-term fixation of femoral stems have focused primarily on press-fit designs often with some form of porous coating for biological ingrowth. Whether this approach will be successful and widely applicable is currently difficult to assess due to a number of issues. Firstly, the early results of contemporary cemented femoral components have improved since the introduction of modern cementing techniques (Cornell and Ranawat, 1986; Harris and McGann, 1986; Roberts, et al, 1986). Secondly, contemporary cementless prostheses may not be optimal in terms of design and in the use of the ingrowth materials, one consequence of which is inconsistency of bone ingrowth (Thomas, et al, 1987; Jasty, et al, 1987.) The early experience with a variety of cementless designs has led to some uncertainty about the relative importance of the many variables that must be tested in an effort to improve the clinical results.
A general conclusion that can be drawn from the previous work is that the clinical results improve with the degree of fit achieved at surgery, indicating that immediate stable fixation is of paramount importance to clinical success. (Ring, 1978; Engh, 1983; Ring, 1983; Itami, 1983; Morsher, 1983; Bombelli, 1984.) It has become clear, however, that certain design strategies for immediate fixation are inferior to others. For example, fixation achieved with long stems that are fully coated with an ingrowth material may produce deleterious proximal bone resorption (Brown and Ring, 1985). It is generally believed that if micromotion occurs between the stem and the bone during weightbearing, fibrous tissue will be formed (Cameron, et al, 1973) which can lead to eventual clinical loosening of the device. Primarily for this reason, the main thrust of the research in the United States has been to achieve primary biologic fixation with bone by the application of porous coating. To date, however, retrieval studies have reported that only a fraction of the acetabular and femoral surfaces achieve bone ingrowth.
Our initial approach to cementless stem design is based on the proposition that a stem shape that closely resembles the anatomy of the femoral canal, particularly in the proximal region, can achieve intimate contact and stability and approximate the load transmission patterns of the normal femur. The fit achieved with such an anatomic design, with the emphasis on maximum fit in certain priority, areas of contact, should result in maximal load transfer to cortical bone and resist not only axial and bending loads, but the important torsional loads as well.
A non-anatomic stem design can result in an apparently stable interface with a benign layer of fibrous tissue. (Kozinn, et al, 1986.) But, if the stem is much more inherently stable due to anatomic fit, and is constructed from an appropriate biocompatible material such as titanium alloy, it may be possible to achieve an interface of bone upgrowth with no interposition of fibrous tissue. (Lintner, et al, 1986; Linder, et al, 1983.)
The purpose of the present invention is to develop a method for the design and evaluation of a prosthesis including the combination of all of the significant elements on a given side of a human joint, for maximum geometric compatibility with the supporting bone structure of the recipient of the prosthesis. The object is to provide in such a prosthesis articulating surfaces and supporting elements therefore in combination to allow press-fit implantation and achieve sufficient stability to induce a stable biologic interface. A further purpose is to establish appropriate design parameters for initial stabilization, and thereby make it so that any ingrowth material at the interface of a joint which is made up of components so designed on both sides of the joint will thereby maximize formation of bone ingrowth and permit better evaluation in the future. An additional object of providing an anatomic design (including a relatively smooth articulating surface) is to increase versatility, whereby downsizing of the supporting elements can be performed to render them suitable for fixation with cement, and thereby to provide a uniform mantle with respect to cortical bone. A further object of the invention is to provide a substantially customized anatomical fit for prostheses for each patient, but to do it in a practically achievable and economical way.